1. Field of the Invention
The present invention relates to a method and system for the regeneration of a nitrogen oxide (NOx) reducing catalyst and a sulfur oxide (SOx) removal catalyst. More particularly, the present invention relates to a method for regenerating both the NOx reducing catalyst and the SOx removal catalyst while additionally preventing sulfur poisoning of the NOx reducing catalyst.
2. Brief Description of Art
Methods for removing contaminants such as NOx from the exhaust gases of diesel engines, gas turbines, and the like without the need to use ammonia have been in development since the middle of the 1990s. One such method is known as a NOx reducing catalyst. A NOx reducing catalyst is a support structure coated with a sorbent material for reducing both carbon monoxide (CO) and NOx emissions. In an oxidation and sorption step, the NOx reducing catalyst works by simultaneously oxidizing CO to CO2 and NO to NO2. The NO2 is sorbed by the sorbent material coated on the catalyst, which is typically potassium carbonate. The CO2 is exhausted out of the stack. When the NO2 is sorbed by the potassium carbonate, potassium nitrites and potassium nitrates are formed.
Since the NOx reducing catalyst can easily be deactivated by SOx and other sulfur compounds found in the exhaust gas, another system known as a SOx removal catalyst is typically arranged upstream of the NOx reducing catalyst, either as a primary SOx removal unit or, more typically, for removing residual amounts of SOx from the exhaust gas. The SOx removal catalyst sorbs SOx from the exhaust gas thereby protecting the NOx reducing catalyst from sulfur poisoning. The SOx removal catalyst is a support structure coated with a sorbent that is effective to sorb SOX from the exhaust gas.
As used herein, the terms “sorb”, “sorbency”, “sorbed”, “sorption”, and the like, indicate either absorbency or adsorbency or a combination thereof. The NOx reducing catalyst can remove NOx from an exhaust gas stream by adsorption, absorption or a combination thereof. Similarly, the SOx removal catalyst can remove SOx by adsorption, absorption, or a combination thereof.
In the traditional system utilizing a SOx removal catalyst and a NOx reducing catalyst, as soon as the depositing capacity of the sorbent material is exhausted, the sorbent material on the catalysts must be regenerated. Regeneration of the sorbent material is traditionally done in situ by isolating the substrate and sorbent material from the exhaust gas flow and contacting the sorbent material with a regeneration gas.
In one system, the regeneration gas contains a portion of molecular hydrogen as the active substance. The remainder of the gas is a carrier gas which consists of steam and may contain small amounts of molecular nitrogen and carbon dioxide. The regeneration gas reacts with the sorbed nitrites and nitrates on the sorbent material of the NOx reducing catalyst to form water vapor and nitrogen which are emitted with the regeneration gas exhaust. Any carbon dioxide present in the regeneration gas reacts with the potassium nitrites and potassium nitrates to form potassium carbonate. As discussed above, potassium carbonate is the sorbent material on the surface of the substrate before the oxidation and sorption step began. The SOx accumulated on the SOx removal catalyst is converted into SO2 and water in the presence of hydrogen in the regeneration gas. In regeneration of the SOx removal catalyst, the catalyst must be reduced (i.e. freed of sorbed oxygen) before the liberation of the sorbed sulfur dioxide can begin.
If the SOx removal catalyst is not fully regenerated at the end of the regeneration, a “puff” of sulfur is often released. Upon re-introducing the exhaust gas into the SOx removal and NOx reducing catalysts, the sulfur puff is entrained into the exhaust gas and carried to the NOx reducing catalyst. As mentioned above, sulfur exposure is detrimental to the NOx reducing catalyst as it destroys the sorption capacity of the NOx reducing catalyst, which cannot be recovered in the regeneration sequence described above.
Therefore, the sulfur puff that occurs during traditional regeneration sequences used in these processes is detrimental to the NOx reducing catalyst. Additionally, a small amount of SOx may also slip-over to the NOx reducing catalyst during the sorption step. The slip-over depends on several factors, including the regeneration and sorption efficiency and capacity of the SOx removal catalyst.
The regeneration sequence traditionally takes place in an oxygen free environment. Additionally, the regeneration sequence should take place in an area isolated from the exhaust gas stream.
In another embodiment disclosed in the art for installations operating at temperatures greater than 450° F., the sorbent material can be regenerated by introducing a small quantity of natural gas with a carrier gas such as steam, to a steam reforming catalyst, and then to the NOx reducing catalyst. In this embodiment, the reforming catalyst initiates the conversion of methane in the natural gas to hydrogen. The conversion is completed over the NOx reducing catalyst.
It should be noted that the SOx removal catalyst utilizes the same oxidation/sorption step and regeneration sequence as the NOx reducing catalyst.
To allow for in situ regeneration without the total disruption of the gas stream flow, the NOx reducing catalyst and SOx removal catalyst are placed in reactor compartments with large dampers at each inlet and outlet. During regeneration, the dampers close, preventing the exhaust gas stream from entering into the reactor compartments. The regeneration gas is then ducted through a distribution system into the compartments to regenerate the sorbent material.
A typical NOx reducing catalyst for a gas turbine of a combined cycle power plant or the like has five to fifteen individually isolatable reactor compartments, 80% of which are in the oxidation/sorption sequence and 20% of which are in the regeneration sequence at any one time. A regeneration sequence typically takes no less than 3 minutes and the oxidation/sorption sequence typically takes no less than 10 minutes, and depends on a variety of factors, including, but not limited to the sorption capacity of the catalysts and the efficiency of regeneration. Accordingly, the efficiency of NOx removal is dependent on the efficiency of regeneration.